I'm using Composer XE 2018 with VS 2015 to create a Fortran DLL. I've added to the project a C source file but in the build process it is simply being ignored. My workaround was to create a C static library project and add the lib file to the Fortran DLL project but I prefer to have them all in one project. Have I missed something?
The separate static library is the way things are meant to be done.
A Fortran project is basically a set of settings for the Fortran tool chain to build something, that Fortran tool chain doesn't understand C source.
Mixed language projects used to exist, but that is now ancient history.
Microsoft did away with the ability to have mixed-language projects in VS.NET 2002.
You'll also want to read https://software.intel.com/en-us/articles/configuring-visual-studio-for-mixed-language-applications